1. [Field of the Invention]
The present invention relates to a material for a light emitting element suitable for a light source for plastic optical fiber communication or more specifically to a material for a light emitting element suitable for a light emitting diode (LED) or a laser diode emitting visible light of 550 to 650 nm band wavelength, and also relates to a method of developing mixed crystals of indium gallium phosphide directly on the GaAs-substrate, the III-V group compound, which method is a crystal growth technique in processing a compound semiconductor, for use in manufacturing the above material.
2. [Description of the Prior Art]
Generally, optical communication involves a plastic optical fiber whose core is composed of methyl polymethacrylate (PMMA) and whose clad is composed of, say, fluorine (F) introduced in the molecular chain of methyl polymethacrylate of lower transmission loss. It is conventionally known that the plastic optical fiber of the above structure provides conspicuously low transmission loss, as shown in FIG. 9, in the 640 to 670 nm wavelength region (hereinafter called 660 nm band wavelength [shaded portion in FIG. 9]) and in the 550 to 600 nm wavelength region (hereinafter called 570 nm band wavelength[.shaded portion in FIG. 9]). The transmission loss is 150 to 280 dB/km for the 660 nm band wavelength, and 120 to 170 dB/km for the 570 nm band wavelength.
In the GaInP system, this light emitting wavelength has a specific correlation to the molar fraction x of GaP in GaInP. The 550 nm and 600 nm wavelengths correspond to the molar fractions of 0.74 and 0.63, respectively, as explained below.
The relationship as expressed by the following equation between light emmitting wavelength of LED (.lambda.p) and bandgap (E.sub.g) is evidenced by the equation 5.1-1 on page 4 of HETEROSTRUCTURE LASERS, H.C. Casey, Jr. et al., PART B, ACADEMIC PRESS, New York, 1978. ##EQU1##
On the other hand, the relationship as expressed by the following equation between E.sub.g and the molar fraction of x of GaP in Ga.sub.x In.sub.1-x P is evidenced by FIG. 5.3-1 on page 16, the disclosure beginning from the first line of page 20 and FIG. 5.3-6 on page 20 of the above reference. EQU E.sub.g (eV)=1.351+0.643x+0.786x.sup.2 (0.ltoreq..times..ltoreq.0.74 ) (2)
The bandgaps E.sub.g with respect to 550 nm (0.55 .mu.m) and 600 nm (0.60 .mu.m) of the wavelengths .lambda.p are calculated using the above equation (1) and each obtained E.sub.g value is substituted in the equation (2) to give the molar fraction x. The results are summarized below.
______________________________________ Wavelength Bandgap Molar Fraction .lambda..sub.p (nm) E.sub.g (eV) x ______________________________________ 550 2.257 0.74 600 2.068 0.63 ______________________________________
The width of the low transmission loss region is broader for the 570 nm band wavelength than for the 660 nm band wavelength. Namely, considering that light emitting wavelength of LEDs has a distribution, an LED of 570 nm band wavelength with broader low loss region can realize optical fiber communication with less transmission loss.
It is clear, therefore, that in optical communication with plastic optical fiber, a yellow-green LED with luminous center wavelength at 570 nm band provides less transmission loss than a red LED with luminous center wavelength at 660 nm band.
GaAsP with a direct band gap (direct transition type for the red wavelength band) and GaAlAs with a direct band gap are conventionally available red LEDs of 660 nm band wavelength. Their luminous efficacies are 0.1 to 0.2% (100 to 300.mu.W in luminance) and 2 to 8% (500 to 2,000.mu.W in luminance), respectively. And conventionally available green LEDs of 570 nm band wavelength are only GaP and GaAsP of indirect transition type whose luminous efficacy is very low or 0.1 to 0.2 % (25 to 50.mu.W in luminance). So, although the conventional green LED is superior in transmission loss by the plastic optical fiber, its low luminance makes it unsuitable to optical communication.
Presently, LEDs with wavelength near 570 nm band (yellow to green) with GaAsP developed on a GaP of GaAs substrate have been sold in the market, although they are exclusively used for display. Since the GaP or GaAsP LED of indirect transition type is poor in luminous output and slow in modulation rate, it cannot be used for the same purpose as a red LED of 660 nm band wavelength. This is why it has not been put into practical use in optical communication.
In view of this, the industry has presented a green LED which is suitable for optical communication and which has overcome the disadvantage of the conventional GaP or GaAsP green LED without sacrificing the advantage of the LED of 570 nm band wavelength. This green LED uses InGaP instead of GaAsP. That is, InGaP layer is developed on a GaAs substrate. Since it has a direct band gap, it provides good luminous efficacy and high luminous output. In addition, its modulation rate is high. Accordingly, it has a wide range of applications from short distance communication by plastic optical fiber (for use in moving vehicles, houses, buildings, etc.) to character displays (large indicator lamps, large display units, etc.) to OA equipment (facsimiles, copying machines, etc.)
In-Ga-P ternary system solution is normally used in growing mixed crystals of InGaP having the same lattice constant as the GaAs substrate. However, since the lattice constant of InGaP of the composition suitable for green LEDs differs from that of the GaAs substrate, it is difficult to epitaxially grow mixed InGaP crystals for green LEDs directly on the GaAs substrate by using In-Ga-P ternary system solution. In the conventional method, therefore, it is necessary to form a GaAsP composition gradient layer for conforming the lattice constant of the GaAs substrate to that of the epitaxially grown InGaP layer.
The typical process of forming the gradient layer is as follows. Firstly, a layer of a material with the same lattice constant as a substrate is grown on the substrate. Then, a layer of material with gradually changing lattice constant is formed on the first layer until the lattice constant becomes equal to that of the epitaxially grown layer or the light emitting layer. For epitaxial growth of a light emitting InGaP layer on a GaAs substrate, a gradient layer of GaAs.sub.y P.sub.1-y is formed on the substrate, with the value of "y" decreased gradually (that is, the content of P is increased gradually.), to fill up the gap of the lattice constant between the GaAs substrate and the InGaP layer.
Material for green light emitting element with GaAs.sub.y P.sub.1-y gradient layer formed on a GaAs substrate is sold in the market as a material for green LED with gradient layer. Compared with the type with no gradient layer on the GaAs substrate, this commercially available material with gradient layer is expensive. Naturally, the resulting light emitting element such as LED or laser diode manufactured therefrom is expensive. Moreover, misfit dislocation due to the gradient layer deteriorates the luminance and the reliability of the element seriously.
From the viewpoint of the method of producing the above material for light emitting elements, III-V group compound InGaP, whose effective forbidden band gap can be increased by selecting appropriate alloy composition, is one of the most important materials for a compound semi-conductor, particularly when the semiconductor is to be used in a visible light region (yellow to green). However, InGaP of the composition corresponding to the visible light region has a high melting point of about 1,450.degree. C. and its dissociation pressure at this temperature is very high or about 32 atmospheric pressure. Consequently, it is difficult to pull up the InGaP alloy-crystals from InP-GaP quasi-binarg solution by using a normal method.
As one of the methods for growing InGaP alloy crystals, epitaxial growth technique can be used, which has been well established on an industrial base. Epitaxial growth technique by which to grow a single crystal on a seed crystal substrate has two types: vapor phase epitaxial (VPE) growth technique and liquid phase growth tehnique. The former technique is for growing crystals by supplying source material from a vapor phase onto a seed crystal substrate, using chemical reactions. The latter technique is for growing crystals by allowing saturated solution of source semiconductor material dissolved at a high temperature in a metal solvent to come in contact with a seed crystal substrate and then cooling it so that the source semiconductor material as a solute is supersaturated. precipitating on the seed crystal substrate. With the vapor phase growth technique, crystals of high purity and therefore of high resistance can be developed. Besides, it is possible to vary the concentration of impurities added as crystals are growing. With the liquid phase epitaxial (LPE) growth technique, on the other hand, since crystals can be developed at a low temperature, it is easy to grow crystals containing components of high vapor pressure. Besides, crystals can be grown within a short period by a simple apparatus. In addition, it is easy to obtain crystals containing large amount of impurities. Needless to say, an appropriate technique should be selected according to the purpose.
Liquid phase growth technique is suitable for growing InGaP crystals, considering its high vapor pressure around its melting point. However, it is difficult to grow, on GaAs as a seed crystal substrate, InGaP crystals of such alloy composition as to emit light in the yellow to green wavelength region. This is due to the lattice mismatch and due to the fact that the seed crystal substrate is soluble in the crystal growth solution
The dissolution of the seed crystal substrate could be prevented by dissolving GaAs to saturation in addition to In, Ga and P in the metal solvent. Since the solubility of GaAs is as large as that of InGaP, however, the resulting crystal would contain considerably large quantity of GaAs, hampering the light emission in the intended wavelength region.
This problem can be solved when a solution of Sn, Ga and InP is made in contact with the GaAs substrate to grow InGaP alloy crystals on the substrate. And yet, since the solution contains smaller numbers of atoms of V group elements than the total number of atoms of III group elements, the GaAs substrate tends to be dissolved in the solution, deteriorating the quality of the resulting crystal grown layer.
Another possible method for solving the above problem is to form a GaAsP buffer layer between the GaAs substrate and the InGaP layer. This method restricts the orientation of crystal face: GaAsP can be grown only on the face (100) of the GaAs crystal orientation. Besides, since the grown InGaP crystal takes over the previously described misfit dislocation, the resulting light emitting diode cannot provide high luminance.
In view of the above, the object of the present invention is to provide a material for a light emitting element most suited for use in optical communication which material is less expensive, more economical and smaller in dislocation density than the conventional material for green light emitting elements with GaAs.sub.y P.sub.1-y gradient layer formed on the GaAs substrate, and a method for crystal growth of the material.